1. Field of the Invention
The present invention relates to projection lenses, and more particularly, relates to projection lenses for use as television projectors.
2. Discussion of Related Art
In recent years, television projectors have been widely used as a display apparatus which magnifies an image and displays the magnified image on a wide screen. Examples of the image treated in such a television projectors include, as well as images displayed on a display unit of a television set, images produced by electronic visual devices, such as VTR, LD and the like.
In order to reduce the size, weight and manufacturing cost of the overall television projector, it is necessary to reduce the size, weight and manufacturing cost of a projector lens used as a component device of the television projector. In the projector lens, it is required to suppress occurrence of various aberrations so as to achieve excellent optical characteristics because the projector lens have a considerably large magnifying power.
A conventional projection lens is constituted by a plurality of (e.g. five or six) lens units so that not only a desired large magnifying power but also excellent optical characteristics can be achieved. In such a constitution, for the power lens unit, which serves to assure the magnifying power and brightness of the formed image and to prevent focal shift, a glass lens element is employed. Also, for the other lens units, aspherical plastic lens elements made of an acrylic resin are employed. The reason for employing plastic lens elements is that plastic lens elements are easy to form and contribute to reducing the weight and manufacturing cost of the overall projection lens. The reason for employing acrylic resin is that the lens element made of acrylic resin exhibits a relatively large Abbe number (i.e. distribution range of wavelengths becomes narrow) compared with lens elements made of plastic materials.
Meanwhile, in recent years, many images treated by television projectors are colored ones. Accordingly, it is important for the television projectors to exhibit smaller aberrations, above all, smaller color aberrations. In the field of television projectors for high-vision television, which requires a high resolution, it is strongly demanded to achieve high optical performances, including a performance of suppressing color aberrations.
However, generally speaking, it is very difficult to completely suppress such color aberrations throughout the entire image projected on the screen through a projector lens because the projector lens consists of a plurality of lens units.
Under the above circumstances, it has been demanded to enhance the optical characteristics in the projector lens.
In this respect, the plastic lens element made of acrylic resin serves to suppress color aberrations effectively to some extent because the lens element made of acrylic resin has a large Abbe number, as mentioned above. However, the extent of the lens element made of acrylic resin is not sufficient for the level demanded.